Peel and cube the apples, avoiding the core.
In a small pot, combine the apple pieces with the brown sugar and cinnamon. Cook over medium heat, stirring often, for 6-8 minutes until the apples begin to break down.
Stir in the cornstarch and continue to cook another 2-3 minutes until the mixture thickens. You are looking for a texture that is in between applesauce and jam.
For the crust, mix together the butter, granulated sugar, and flour. Add a tablespoon of water and continue to mix until a very crumbly dough forms. If needed you can add another tablespoon of water.
Divide the dough in half. In a square baking pan covered with parchment paper, press half of the dough into the bottom. Bake at 350°F for 12 minutes.
Then spoon the apple pie filling over the partially baked crust. Crumble the remaining dough over the top. Bake for an additional 30 minutes, until the crumb topping begins to brown.
Allow to cool before cutting into squares.